1. Field of the Invention
This invention relates to a diversity receiver utilizing a plurality of antennas and, more particularly, to such a diversity receiver employing a phase noise restraining technique for controlling the phase noise due to the phase non-contiguity of the receiving wave seams that are inevitably produced at the time when the receiving wave is switched.
2. Description of the Related Art
One type of prior art diversity receiver is the antenna switching type. This type of diversity receiver comprises a plurality of antennas, an antenna switching circuit and a receiving part and functions to select the antenna whose receiving electric field intensity is high by means of an antenna switching circuit and then introduces the received wave to the receiving part. Therefore, the receiving part can be constructed as a single system, and can result in a diversity receiver of low cost with a reduced number of parts. As an example of the diversity receiver of this kind of antenna switching system, the diversity receiver which is disclosed in Patent Kokai (Laid Open) No. Sho 62-47223 is known. According to said diversity receiver, a switch threshold value is renewed on the medium and high receiving electric field intensity levels, to select the antenna whose receiving electric field intensity is higher. The switch threshold value is fixed at a prescribed value on the low receiving electric field intensity level and antenna switching is repeated many times at the time when the receiving electric field intensity of the plurality of antennas happens to be lower than the lower limit. Switching is terminated at the time when any of the antennas has exceeded the lower limit. The diversity effect is heightened in conformity with the variations in the receiving electric field intensity.
In the case of the aforementioned diversity receiver, antenna switching is carried out frequently on the lower receiving electric field intensity level, with a result that phase non-contiguity arises at the seam (juncture) of the receiving signals due to antenna switching and with a result that is spike noise is produced in the case of the demodulated signal. Since this low receiving electric field intensity level reflects the state in which the quality of the communications is unsatisfactory, the demodulated signal is cut off by a squelch circuit (or a muting circuit), thereby posing no serious problem. On the medium and high receiving electric field intensity levels, on the other hand, phase non-contiguity (phase shift) is inevitably produced at the juncture between the receiving wave prior to switching and the receiving wave after switching due to the antenna distance or the directional character of the receiving wave, etc., even if the switching frequency is low, with a result that phase noise appears in the demodulated signal (such as the sound signal). In the case of a portable radio or radio receiver, where the S/N (signal to noise) ratio is comparatively low, this phase noise is not easily detected as it gets mixed with the external noise. In the case of the receivers of a wireless microphone system or data transmission receivers, etc., where the S/N ratio is high, undesirable foreign noises or signal bits are produced, thereby posing a serious problem in the control of the phase noise.
On the other hand, there has been known a diversity receiver of the synthesizing type, as is described in U.S. Pat. No. 4,293,955, whereby either a signal of zero phase delay or a signal whose phase delay is 180 degrees is switched and synthesized on the basis of the wave that has been received from the second antenna as compared with the wave that is received from the first antenna, with the synthesized wave being introduced to the receiving part of a single system. The reason why a signal whose phase delay is 180 degrees is prepared is that the intensity of the synthesized signal becomes zero at the time when the wave received from the second antenna is shifted by a phase of 180 degrees as compared with the wave that is received from the first antenna. Accordingly, a signal of the reverse phase is prepared beforehand on the basis of the wave that is received from the second antenna and either one of the signals of the regular phase and the reverse phase is switched and added to the received wave from the first antenna so as to elevate the signal intensity of the synthesized wave. Even in a diversity receiver of such a synthesizing system where there is a switching circuit, however, there is produced a phase shift between the synthesized wave prior to switching and the synthesized wave after switching, with a consequence that phase noise inevitably appears in the demodulated signal.
In either of these systems, the diversity receiver of the antenna switching system having the receiving part of a single system has been abandoned due to the extreme difficulty of controlling this phase noise in the past, with a result that the expensive space diversity receiver having the receiving part of two systems has inevitably been put to commercial use.